There is one passage the relevance of which I think is underappreciated in the prolepsis discussion. This below from fragment 5 of Diogenes of Oinoanda gets referenced frequently in regard to Epicurus' canon in general, but I wonder if it not a specific reference to the function of prolepsis:
QuoteFr. 5
....Now Aristotle and those who hold the same Peripatetic views as Aristotle say that nothing is scientifically knowable, because things are continually in flux and, on account of the rapidity of the flux, evade our apprehension. We on the other hand acknowledge their flux, but not its being so rapid that the nature of each thing [is] at no time apprehensible by sense-perception. And indeed [in no way would the upholders of] the view under discussion have been able to say (and this is just what they do [maintain] that [at one time] this is [white] and this black, while [at another time] neither this is [white nor] that black, [if] they had not had [previous] knowledge of the nature of both white and black.
When you're living in an age when most every educated person would be aware that Heraclitus has said everything is in such flux and flows so fast that it is impossible to apprehend anything at all, you need a description of the process by which you DO apprehend things and make sense of them.
I wonder if prolepsis then might best be understood as Epicurus' answer not just to Plato and Aristotle, who were themselves apparently responding to Heraclitus by postulating that there are true forms or essences (neither of which exist).
Epicurus' prolepsis provides the foundation of an answer to Heraclitus' flux challenge in a natural faculty, just like pleasure and pain, to how we actually understand the things around us without reliance on forms or esences which do not exist, or on preexisting innate ideas from a time before birth. In providing a theory of understand the assembly of knowledge, it is parallel to atomism in providing a theory of physics.
Even as to the title we generally give to Lucretius' poem, how would we know what a "thing" is, or distinguish one "thing" from another, if we did not have a faculty which continuously organizes the raw data from the senses into something intelligible?
As I understand it there are not many reliable quotations from Heraclitus available, but those that do make it clear that this "flux" problem demanded a real-world answer.